The present invention relates to a display device adapted for a digitally detecting and displaying changes in a quantity to be measured such as for a fuel meter for an automobile, wherein flicker of display attributable to fluctuations in fuel level caused by vibrations is prevented, so that electronic display may be provided with good response.
In order to electrically detect and display a quantity to be measured which irregularly changes by external factors such as residual amount of fuel in a fuel tank, there is known a method which incorporates a flow rate sensor and a counter. The flow rate sensor arranged in a fuel passage generates a pulse signal upon every flow of a predetermined amount of fuel. The pulse signals are supplied to the counter which has a preset count corresponding to the initial fuel level and which counts down upon reception of each pulse signal. With this method, flicker attributable to level fluctuations due to vibrations does not occur and display of a residual amount of fuel can be performed with a relatively high precision. However, with this method, since the detection error of the flow rate sensor is accumulated, a flow rate sensor of high precision must be used. Furthermore, presetting of the initial fuel level, more specifically, setting of the fuel amount after a filling up which is the sum of a residual fuel amount and a newly filled fuel amount largely depends upon the display precision of fuel supply equipment and other factors. This has impaired correct presetting of the initial fuel level. Another method is also known which incorporates a float and a unit having a sliding resistance in the fuel tank. According to this method, the resistance is varied according to the vertical movement of the unit with the fluctuations in the position of the float or fuel level. Then, fluctuations in the voltage dividing level are detected to directly detect the fuel residual amount. The voltage dividing level is A-D converted to allow digital display. However, with this method, fluctuations in the fuel level caused by vibrations of a vehicle are immediately detected as changes in the voltage dividing level, resulting in more frequent flicker of the display. For allowing digital or analog display of the vehicle speed, a mechanism is generally adopted wherein a signal of a frequency corresponding to the rotational frequency of the vehicle wheels or transmission mechanism is generated. These signals are counted with a predetermined period to transfer the speed data to the display device. However, the rotational frequency of the vehicle wheels frequently irregularly changes due to the non-smooth surface of the road or the like. Naturally, the signal corresponding to such frequency fluctuates, so that the digital display is also subject to flicker and this indication is very hard to read.
As a means for preventing flicker of the display due to such external factors, a method has been proposed wherein, only when the same change in the large or small value between the display data corresponding to the present display value and the sampling data formed by the output of the sensor continues over a predetermined period of time, is the present display value changed by the changed value as a current measuring value. Although this method may prevent unnecessary frequent fluctuations in the display, it is simultaneously subject to the problem of slow response to actual changes in the fuel level. More specifically, with this method, if sampling data having the reverse relation to the display data is input even once due to external factors during a predetermined period of time for discriminating the large or small relation between the display data and the sampling data, the display may not be updated by the immediately reverse change of the sampling data due to external factors although the sampling data is changed by the actual change in the fuel level.